Update syntax

Author: simoncozens

Makefile

@@ -36,7 +36,7 @@ test: build.stamp
 
 proof: venv build.stamp
 	which diff3proof || (echo "diff3proof not found. Please install it with 'cargo binstall diffenator3'." && exit 1)
-	TOCHECK=$$(find fonts/variable -type f 2>/dev/null); if [ -z "$$TOCHECK" ]; then TOCHECK=$$(find fonts/ttf -type f 2>/dev/null); fi ; . venv/bin/activate; mkdir -p out/ out/proof; diff3proof $$TOCHECK -o out/proof
+	TOCHECK=$$(find fonts/variable -type f 2>/dev/null); if [ -z "$$TOCHECK" ]; then TOCHECK=$$(find fonts/ttf -type f 2>/dev/null); fi ; . venv/bin/activate; mkdir -p out/ out/proof; diff3proof $$TOCHECK --output out/proof
 
 images: venv $(DRAWBOT_OUTPUT)